Air conditioning apparatus



Sept. 23, 1941. imam 256,566

AIR CONDITIONING APPARATUS Filed May 3, 1939 2 Sheets-Sheet 1 ags ROOM TEMP.

PERCENT OUTDOOR OUTSIDE MIXED All? TEMP.

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J. MOELGIN 2,256,566

AIR CONDITIONING APPARATUS Filed May 3, 1959 2 Sheets-Sheet 2 9 9- 4 I 9 ,ss 54 5,4 56

PEST/P/CTOP VALVE 35 WMQ Airmen/5y:

Patented Sept. 23, 194i AIR CONDITIONING APPARATUS John McElgin, Philadelphia, Pa., assignor to John J. Nesbitt, Inc., Philadelphia, Pa., a corporation of Pennsylvania Application May 3, 1939, Serial No. 271,551

4 Claims.

The present invention relates to the art of air conditioning and more particularly to a control system for regulating the amount of outdoor air admitted to the air conditioning unit.

1 In buildings or other enclosures where large numbers of people are apt to gather, it is often desirable to increase the germicidal effects and the vitality of the air by introducing into the building preferably through the air conditioning units, a predetermined minimum amount of fresh air. However, in accordance with the present invention, I have found that enhanced results are obtainable along the lines noted by introducing a fixed percentage of outdoor air with respect to indoor or recirculated air. This percentage might be as high as 50% or more. As the percentage is increased, the heating facilities of the building must be increased in a corresponding degree.

In most ventilating systems of this general type and particularly in unit ventilating systems, an initial adjustment of the outdoor and indoor air dampers practically never insures that the proportions of the indoor and outdoor air will remain constant. One reason for this The primary object of the invention is to maintain a constant proportion, either by weight or volume of the inside and outside air within an enclosure regardless of the direction and force of the outside wind and regardless of the temperature of 'theoutside air.

Another object is to provide a system of the character described, in which asingle outdoor deficiency is that constantly changing wind conditions may cause a system to introduce a greater or less quantity of outdoor air than desired. For example, on the windward side of the building, a ventilating system with a fixed damper position will tend to handle more outdoor air than is desired, whereas on the lee side of the building, the tendency is to handle a less quantity of air than is desired. As a matter of fact, it is often possible, because of high wind velocities, for a quantity of outdoor air equal to or greater than the system capacity to enter through "a restricted outdoor intake. This results (in unit ventilators) in the discharge of air through the indoor air intake-a condition that is commonly called blow through. Another reason is due to the changes in outside temperature, For example, as the outdoor air becomes colder and assuming that the thermometer of the ventilating system, which controls the damper, is set for a predetermined mixed air temperature,'the lowered temperature of the outdoor air will cause the thermometer to close the damper, thus reducing or entirely preventing the entry of fresh air into the building.

It is apparent that, under both of the conditions noted, i. e., wind effects and low outside temperatures, the recirculated air within the building loses its germicidal and vitality qualities due to the absence of fresh air.

temperature compensating element is employed in connection with a multiple arrangement of ventilating units.

These objects are attained in brieflb'y adding a compensatory feature to the thermometer which is actuated by the temperature of the' mixed air'and in response thereto operates the dampers which control the proportion of fresh and recirculated air. In case this compensating element operates to open the dampers for air at a lower temperature in order to maintain a fixed proportion between the outdoor and indoor air, thus tending to cool off the room, a control is exercised by a second thermometer which serves to increase the heat given off by the radiators.

Other objects and features will be apparent as the specification is perused in connection with the accompanying drawings, in which Figure 1 is a diagrammatic view of a combined ventilating and heating system improved in accordance with the present invention.

Figure 2 is a side view of a practical installation of the system shown in Figure 1.

Figure 3 is a chart useful in explaining the principles of the invention and showing the various relations between the temperatures and volume of outdoor air.

Figure 4 is a view, partly in section, of a differential thermostat which may be employed to advantage in carrying out the present invention.

Figure 5 is a side view of an installation utilizing the thermostat shown in Figure 4.

Figure 6 is a diagrammatic view illustrating a single readjustment unit or element providing outdoor temperature compensation to a plurality of ventilating systems.

Figure 7 is a fragmentary view of a system similar to Figure 6, except that the separate readjustment or compensatory pressure line has been eliminated and the function of this line is carried out through the main pressure line. To this extent, the systemshown in Figure 7 requires less conduits than that illustrated in Figure 6.

Referring more particularly to Figure 1, numeral I represents the casing of an air conditionture.

ing unit, the lower portions of which terminate on one side in an indoor or recirculated air inlet 2 and ,at the other side in an outdoor or fresh air inlet 3. The casing is restricted at the middle,

as indicated at 4 and there is a wall or partition portion 5 which divides the lower portion of the casing into two passageways. A damper 6 of arcuate configuration is adapted to swing on the pivot, 1 and to close either one of the passageways leading to the inlets 2, 3. This damper is actuated through a linkage 8 and a crank 9 which'is moved by a. damper motor or engine I0, as will be explained hereinafter.

The upper portion of the casing I is enlarged in order to encompass a blower system constituted of a pair of centrifugal blowers of the volute chamber type, as seen more clearly in Figure 2, and actuated by a motor l2. Immediately above the blowers and positioned so as to respond to the blower discharge, there is a thermostat indicated generally at l3.

The thermostat is constituted of a sleeve l4.

preferably of brass which surrounds a rod l5, preferably of invar metal and rigidly secured together at one end by a disk IS. The sleeve 4 is secured to a circular plate l1 ofmetal which carries a support l8 provided with a pivot l9 at one end. The end of the rod l5 remote from the disk l6 projects through a large opening in the plate I1 and terminates 'in a thumb screw 20. A bar 2| is arranged to pivot at l9 and has an opening through which the rod I5 projects. This bar carries a nozzle 22 which forms a terminus of a conduit system represented in part by the pipe 23. v

The bar 2| also has secured thereto a. pivoted lever 24 which has an opening to receive the shank portion of the thumb screw 20 and is adaned to bear against the opening in the nozzle 2.

The plate |1 carries at its lower end a transverse extension 25 which is provided with an upright 26. There is a bellows 21 positioned between the upright 26 and the bar 20 and also a compression spring 28 between the bar 2| and the plate H. The interior of the bellows 21 communicates through a pipe 29 with a thermostat 30 placed preferably in the outdoor air inlet 3'. The pipe 29 is usually designated a readjustment line. This thermostat may be ofany y which will cause the pressure within the pipe 29. and, therefore, within the bellows 21 to vary in accordance with changes of the outdoor tempera- Thus, the thermostat'30 may be of the pneumatic or of the volatile liquid typ'e The operation of the system described up to this point will now be considered. Assume that the position ofthe damper isas illustrated so that a mixture of indoor and outdoor air-is presented to the blowers ll. As the temperature of this mixed air becomes higher due perhaps to higher temperature of the outdoor air, the brass sleeve 14 of the thermostat l3 will expand relative to the rod 15 and, since the ends of the sleeve and rod are connected together at l6, the

plate 11 is moved to the left with respect to the end of the rod |5 remains stationary due to the fact that it is made of a metal which does not expand upon being heated, it is apparent that the lower end ofthe bar 24 will fulcrum on the end of the rod 15 and tend to move against the nozzle 22 to close the opening therein.

The pipe 23 to which the nozzle 22 is connected is supplied with fluid under pressure, as will be presently explained, and since the opening in the nozzle'is effectively covered by the bar 24, none of the fluid will leak at the nozzle and pressure within the pipe 23 will be at a maximum. Thisincrease in pressure within said pipe will be sufficient to actuate the damper motor III which, in turn, would swing the damper 6 to the left, thus permitting more outdoor air to flow through the inlet 3 and into the blower system.

In the event that the air atthe blower discharge becomes too cool due to an over-abundance of outdoor air, the sleeve l3 will contract and the operation of the thermostat will be reverse to that explained immediately above. Thus, the bar '24 will again be fulcrumed about the lefthand end of the rod l5 and would move to the left to uncover the opening in the nozzle 7 22. The air or other fluid in the pipe 23 would then leak at the nozzle, causing a reduction in pressure within the pipe and further causing the motor I0 to swing the damper 6 to the right to close off the outdoor air and to admit more indoor or recirculated air. The purpose of the thermostat 30, also of the bellows 21 and spring 28 will be' explained after the entire system has been described and when the complete operation is being considered.

Above the thermostat 13, there is a radiator 3| composed of a number of parallelly disposed pipes 32 and preferably provided .with transversely extending radiating fins 33, as seen more clearly in Figure 2. The radiator is provided with steam from the pipe 34 which is under the control of a so-called modulating steam valve 35. A description of this valve is unnecessary in that they are well known in the art and it is sufiiin the fluid pipe 36 which is connected to the valve 35.

Directly above the radiator 3|, i.'e., so as to be responsive to the heat generated thereby, there is a thermostat 31 of practically the same type as the thermostat 13, except that thermostat 31 does not employ the bellows 21 'nor the compression spring 28. However, the thermostat 31 utilizes the sleeve l4 and an interior rod l5 of metals having different coefiicients of heat expansion, as in the case of thermostat l3. Moreover, the thermostat 31' employs a pivoted bar 24 which is adapted to close or uncover the opening in the nozzle 22, depending upon the position of the'bar. This nozzle is connected to a fluid pressure pipe 38 which is similar to pipe 23 of the other thermostat.

For supplying the system with fluid pressure, I provide a compressor 39 of any'suitable and well-known type and actuated by a motor indicated at 40, which fills a tank 4| with air under pressure. From the tank, a conduit is taken through a valve 42 into a line or pipe .43 through a pneumatic thermostat 44, thence through a pipe 45 to connect with the pipe 23. The thermostat 44 may be placed at any position in the room or building and it is for the purpose of introducing an additional check or control on the fluid pressure in accordance with room temperature. A branch line or pipe 46 is taken through a restrictor valve 41 of vwell known type, and beyond this valve, the pipe connects to thepipe 36 referred Operation of the entire system As stated hereinbefore, the purpose of the present invention is to assure the introduction of a predetermined minimum amount of fresh air into a building, regardless of the temperature of the outdoor air and regardless of the velocity and direction of the wind. In particular, the purpose is to introduce a fixed percentage of outdoor air with respect to the indoor or recirculated air. It was pointed out that, under certain conditions, it is desirable that the percentage of outdoor air shall not fall below a given amount in order to maintain the germicidal effects and the vitality of the inside air.

Assume that the outdoor inlet 3 is on the windward side of a building and that the damper 6 is fixed. In other words, there is no thermostat I3 to regulate the position of the damper. If a strong wind were to blow against the inlet 3, it is apparent that a greater proportion of the cooler outside air will be handled by the blowers II which will tend to cool the room or enclosure inlets otherwise compensated for by additional heat from the radiator 3I. However, by providing the thermostat I3 which regulates the position of the damper 6 in accordance with the temperature of the mixed air given off by the blowers, it is impossible for increased wind effects to increase the proportion of the iresh air relative to the indoor or recirculated air. Clearly the thermostat I3 responds to the temperature of the mixed air, and since this temperature is proportional at least in part to the quantity of fresh air being drawn through the inlet 3, the thermostat would immediately detect an increase of outdoor air brought about, for example, by increased wind pressure, assuming that all other factors remain substantially constant.

The sleeve I4 would, therefore, contract under these conditions and which, in turn, would cause the bar 24 to move away from the nozzle 22, as explained hereinbefore, thus reducing the pressure in the pipe 23 and causing the motor I to swing the damper 6 to the right. It is apparent that the thermostat I3 may be adjusted, for example, by turning the thumb screw 20, or it may be designed to introduce a predetermined or minimum percentage of outdoor air with respect to the indoor or recirculated air into the room or building. A typical percentage is 50%,. y

Now assume that the temperature of the outdoor air decreases, it is apparent that the thermostat I3 which responds to the temperature of the' mixed air given-01f by the blowers II will tend to swing the damper 6 to the right, depending-.upon the reduction of outdoor temperature.

'It is quite conceivable that, if the outdoor temperature were sufficiently low, the damper 6 would entirely cut off the inlet passageway 3 to the outdoor air.

Accordingly, in order to assure a fixed per- .centage of outdoor air with respect to indoor air under conditions of changing outdoor tem-,

peratures, it is necessary to change the control point of the thermostat I3 in accordance with these temperature changes. The figures given in the chart on Figure 3 are illuminating in this respect. Thus, assume that it is desirable to maintain a percentage between the outdoor and indoor air of 50% and that the temperature of .the outdoor air increases from 0 to 60, the mixed air temperature will vary from to 65.

It follows from this that the control point of the thermostat must be changed accordingly in order to follow the changes in the mixed air temperatures to assure a fixed and predetermined radiator 3I will become less.

per cent between the outdoor and indoor air.

In general, this change in control requires a higher control point as the outside temperature increases. The change in control is' brought about by the use of the thermostat 30, positioned in the inlet 3 which affectsthe length of the bellows 27. As the temperature of the outdoor air decreases, the thermostat 30 causes the bellows 21 to contract and this, in turn, causes the nozzle 22 to approach the bar 24, and accordingly, less expansion of the sleeve I4 is necessary to move the bar to such a position as to close the nozzle 22. In the event that the temperature of the outdoor air increases, the thermostat 30 causes the bellows 21 to expand, which moves the bar 2I to the right against the spring 28, thus moving the nozzle further away from the bar 24. Under these conditions, a greater expansion of the sleeve I4 would be required to cause the bar 24 to close the opening in the nozzle 22 and the reverse is true insofar as the contraction of sleeve I4 is concerned. It has been pointed out that the operation of the motor I0 and the position of the damper 6 depends on the pressure within the pipe 23 which, -in turn, is controlled by the amount of fluid which leaks from the nozzle 22.

It is apparent that, as the outside temperature increases as indicated on the chart in Figure 3, the thermostat 3B and bellows 21 operate in such a manner as to increase the control point of the thermostat, as is suggested in the third column of the chart. This change in control point assures that there will always be a fixed percentage of outdoor air, for example, 50%, as indicated on the chart with respect to the air mixture contained within the unit.

Obviously, as the temperature of the outdoor air decreases, assuming that the percentage between outdoor and indoor air is maintained substantially constant under the conditions noted, the temperature of the mixed air reaching the If the temperature of the room is to be maintained substantially constant, for example, 70, as indicated in the second column of the chart, it will be necessary to increase the heat generation at the radiator. The amount of steam admitted to the radiator is controlled by the valve 35 which, in turn, is controlled by the pressure'of the fiuid in the pipes 36 and 38. As stated hereinbefore, the

thermostat 31 is similar to the thermostat I3v produces an increase in pressure in the pipe 38.-

This change in pressure transmitted through the pipe 36 causes the modulating valve 35 to cut down the steam supplied to the radiator.

On the other hand, as the outside temperature becomes lower and the percentage of outdoor air relative tovthe indoor air is maintained at a fixed value, the thermostat 31 will detect a lower temperature at the radiator. perature will cause the sleeve I4 of the thermostat 31 to contract, which in effect moves the bar 24 away from the nozzle, permitting fluid to This lower temescape and thereby 'reducing the pressure in the pipes 38 and 36. Thus, the modulating valve 35 is actuated and serves to increase the supply of steam to the radiator.

It will be noted that the room thermostat 44 is in series with the pipes 43 and 45, and can of its own'account, depending on-room tempera-' at the blow'erdischarge, is well adapted to provide an efficient air cooling control for mild weather or summer use. One of the possible functions in this connection is that, at a predetermined outside temperature, the control point of the thermostat l3- may be set, for

example, at 60 and left there, regardless of increases of outside temperature. If 50% outdoor air were introduced, then at 50 and above outside temperature, the control point would be 60. This would provide all outdoor air at a time when the maximum cooling is required." In other words, the readjustment of the control, as exercised by the thermostat 30, would proceed in the regular manner below this point, but when the temperature of the outside had reached 60, the control would be removed and all outdoor air, i. e., to the total exclusion of recirculated air, would be introduced. This adjustment, therefore, avoids mixing indoor and outdoor air when greater cooling Power is necessary. This modified operation is desirable where the percentage of outdoor air introduced as a m-inimum is small (25%) and in mild weathen'when the fixed mixture does not provide adequate cooling =power.

Figure 2 shows the application of the system, explained in connection with Figures 1 and 3, to a practical installation. The parts of theunit are contained within a casing 48 of metal having 1 an inlet for the indoor recirculated air at the bottom, as indicated at 49. The inlet 50 for the outdoor air is also at the bottom of the casing on the opposite side from the inlet 49. The inlet 50 is conveniently positioned at an opening in the wall 5| of the building, The discharge opening for the unit is shown at 52. The remaining elements illustrated in Figure 2 are given the same reference characters as the corresponding elements shown in Figure 1. As pointed out in the latter figure, the thermostat I3 serves to maintain a fixed percentage of outdoor air to indoor air, regardless of wind pressure conditions, but assuming that the outdoor air remains at a constant temperature. The thermostat 30 serves to regulate the control point of the thermostat l3 in accordance with the changes in outside temperature in order that the thermostat will maintain the proper position of the damper to assure a fixed proportion between outdoor and indoor air.

In Figure 4, I have disclosed a modified form of thermostat which compensates itself' for changes in outside temperature. Therefore, this thermostat, as illustrated, can be substituted for the element l3 shown in Figure l. The modified'thermostat is carried on a U-shaped metal frame 53 having openings 54 in the upstanding legs which loosely receive a rod 55, preferably of invar and a rod 56, preferably of brass. A brass sleeve 51 surrounds the rod 55 and is adjustably is fixed in space.

the righthand wall of the envelope.

peratu're increase.

secured thereto at one end, as indicated at 58. This adjustment may take the form of threads provided on the rod 55, which screw into a threaded opening in a boss 59 secured to the lefthand end of the sleeve 51. At the opposite end, the rod 55 is provided with a knurled thumb nut 60. It will 'be noted that the righthand end of the rod 55 terminates at a position immediately adjacent the extension of the valve 64 which pro-' jects through the opening 63. The space between the rod 55 and this extension may be ad-; justed by turning the nut 60.

Surrounding the brass rod 56, there is a sleeve 6l preferably of invar and secured to the rod at one end. The sleeve is also rigidly secured to the upright 53. The rod 56 carries at its lefthand end and positioned within theU-shaped member 53 a hollow metal envelope 62. The lefthand wall of the envelope 62 has an opening 63 therein, which is flared outwardly, as indicated at 64, and serves as a seat for a conical valve 65. There is a spring 66 positioned between the valve and A pipe 61 carrying fluid under pressure terminates within the chamber of the envelope. The other end of the pipe is connected to a source of fluid under pressure and also to the damper motor [0.

Let us'assume that the sleeves 51 and 6| are both subjected to the same increased temperature. The rod 56 will expand relative to the sleeve 6| due to their different coefiicients of expansion and the rod will move the compartment 62 to the left, since the righthand end of the rod The sleeve 51 will likewise expand relative to the rod 55 due'also to diiTerence in coefiicients of expansion. The righthand end of the sleeve 51 is fixed in space and consequently the lefthand end of the sleeve will draw the rod 55 to the left, maintaining the same space between the righthand end of the rod and the lefthand end of the valve extension. In other words, the entire structure contained within the frame 53 would be moved to the left as a unit when the sleeves 51 and 6| are subjected to the same tem- However, if the sleeve 51- and the contained rod 55 were cooled, leaving the sleeve 6| and its contained rod 56 at a predetermined temperature, a difierential expansion effect is produced between the rod 55 andthe valve extension 64. The rod 55 will move to the right under these conditions and, inasmuch as the compartment 62 remains stationary, the rod will strike the valve extension and unseat the valve against the compression spring 66. The opening in the compartment 62 will, therefore, become uncovered, permitting fluid to escape and thereby reducing the pressure in the line 61." This reduction of pressure is suificient to operate the damper motor.

Figure 5 shows a practical method of utilizing the thermostat shown in Figure 4. It will be noted that one end of the thermostat projects outside of the casing 48 where it will be responsive to the temperature of the fresh air being drawn through the inlet 50. The other end of the thermostat is mounted within the casing and operates in exactly the same manner as the thermostat I3 which is described in connection with Figure 1. The portion of the thermostat 53 which projects outside of the casing serves to adjust the control point of the portion which is contained within the casing. Thus, as the out side temperature increases, for example, in'the order set forth in the first column of the chart in Figure 3, the control point of that portion of the thermostat within the casing is increased by the differential effect ofv that portion which is outside of the casing in accordance with the temperatures set forth in the third column of the chart. Under these conditions, the thermostat 53 serves the same function as the combined thermostats Hand 30 in Figure 1 to provide a fixed proportion of fresh air relative to the recirculated air which passes into the unit. I

In Figure 6, I have shown the application of 0 my invention to a group of ventilating units, each of which employs a thermostat I3 and having the control point varied in accordance with the outside temperature by a single thermostat 68. The thermostat 68 is connected by a pipe 69 to a pressure-reducing valve of any suitable and well known type. From the vlave 10, the fluid is conducted through a pipe H which has a branch line 72 connected to a bellows 21 (Figure 1) of the thermostat I3. Thus, the pipe H and the bellows 21 serve as a means for readjusting the control point of the thermostat l3 of each ventilating unit. The pressure in the pipe H is adjusted in accordance with outside temperature by the vlave 10 which responds to. the expansions and contractions of the thermostat 68. The latter'may be of a volatile liquid or pneumatic type and is placed where it can be affected by the fresh or outdoor air.

In this figure, the pressure line which actuates the damper motors I0 is taken through a pipe 13 and then to the branch pipe 14, one branch of which passes through the room thermostat 44 and the other is connected to the nozzle 22 (Figure l) of the thermostat I3. A pipe 43 connects the other side of the thermostat 44 to the reservoir 4|.

The operation of this system is somewhat similar to that explained in connection with Figure 1. However, in Figure 6, the master outdoor air thermostat 68 varies the air pressure in the line H which, in turn, readjusts the control point of all of the thermostats 13. A considerable saving of apparatus is effected by using a single outdoor thermostat to perform this pur- 45 pose. It will be noted that the room thermostat 44 serves to control the pressure of the fluid in pipes l4 and 73 and is, therefore, adapted to control the operation of the damper motor 40. The latter is also under the control of the ther- 5O mostat l3 which, as pointed out in connection with Figure 1, operates in response to changes of temperature to allow fluid to leak at the nozzle 22.

It is evident that the damper motor can be so mechanically adjusted as to power that its control will be invested first in the thermostat- 44 which may cause the damper to remain closed until the room temperature comes up to a given value, after which the control of the motor is restored to the thermostat l3. The latter would then cause a movement of the damper to provide a fixed relation between the outdoor and the indoor air, as explained hereinbefore. During the time that the damper motor is under the conexcept that the function of. the readjustment line H has been taken over by the line 43. Thus, the thermostat 68 is connected to a reducing valve 15 inthe line 43, and variations inthe temperature to which the thermostat 68 is subjected are carried through the line 43 and the line 16 which communicates with the bellows 21 (Figure l) of the thermostat 3 I. For some pneumatic thermostats, the pressure variation permissible in the line 43 without disturbing the control point of the room thermostat 44 would be limited to approximately thirteen to eighteen pounds and for this reason the thermostat 3| would have to be carefully constructed and adjusted. However, if the thermostat 3| were replaced by a pneumatic thermostat of well-known type which operates on the reducing valve principle, wider variations in the main pressure line 43 would be permissible without affecting the control point of the room thermostat 44. It is apparent that the system shown in Figure 7 is simpler than that illustrated in Figure 6 in that the pressure readjustment line H has been eliminated. The pressure variations which normally occur in the line 'H in response to the temperature variations afiecting the thermostat 68 now take place in the main line 43.

From the foregoing, it is evident that while I have described the thermostats l3 and 31 as taking the form of expansion units, these members may be replaced by a thermostat of the volatile liquid type or the pneumatic type. Moreover, these thermostats need not operate on the socalled leakstat principle, in which the pressure in the fluid system is changed in order to operate the motor Ill and the valve 35, but instead an electrical arrangement might be employed. Any suitable electrical control could also be employed for changing the control point p of the thermostat l3 in accordance with the outdoor temperature variations, to which the thermostats 30 (Figure 2) and 68 (Figure 6) respond.

It will be understood that I desire to comprehend within my invention such modifications as come within the scope of the claims and the invention.

Having thus fully described my invention, what I claim as new and desire to secure by Letters Patent is:

1-. A ventilating system including a casing having a recirculated air inlet, an outdoor air inlet and a mixed-air outlet, the recirculated air being of substantially constant temperature, an adjustable damper mounted across said inlets, and means operated by fluid pressure for controlling said damper to admit outdoor air in a fixed proportion with respect to the total air passing through said outlet, said means including a pipe line containing fluid under pressure, a damper motor connected to said line, means including a thermostat responsive to the temperature of the air mixture within the casing, a valve in said line through which fluid can escape, said valve being controlled by said thermostat, and a thermostat for changing the control point of the first-mentioned thermostat in accordance with the temperature of the outdoor air, said means serving to vary the pressure of the control system in accordance with any departure in the proportion of outdoor air from the fixed proportion whereby the damper motor eifects a corresponding change in the position of the damper to maintain a desired percentage of outdoor and recirculated air.

2. A ventilating unit comprising a. casing having a recirculated air inlet, a fresh air inlet and a mixed air outlet, the recirculated air being of substantially constant temperature, means includin'g a thermostat responsive to'the temperature of the mixed air within the casing for maintaining a fixed proportion of fresh air and recirculated air passing through said inlets, an

\ additional thermostat for modifying the control point of the first-mentioned thermostat and respons ive to the temperature of the fresh air, and

means for bringing the temperature of the mixed at back to normal after said first-mentioned means has operated to maintain said fixed proportion of fresh and recirculated air whereby substantially constant temperature, an adjustable damper mounted across said inlets, means including a thermostat positioned exterior to the casing for controlling said damper to admit only recirculated air until the mixed airtemperature reaches a predetermined value, means including a thermostat within the casing for changing the adjustment of the damper when the mixed air temperature reaches said predetermined value to provide a fixed proportion of fresh air relative to the total air flowing through said outlet, and means for changin -the control point of said last-mentioned thermostat in accordance with changes in the fresh air-temperature.

4. In a ventilating system, a plurality of casings having a recirculated air inlet, a fresh air inlet and a mixed air outlet, an adjustable damper mounted across said inlets, means including thermostats positioned exterior to the respective casings for controlling each damper to admit only recirculated air until the mixed air temperature reaches a predetermined value, means including a thermostat within said casing for changing the adjustment of each damper when the mixed air reaches said predetermined value to provide a fixed proportion of fresh air relative to the total air flowing through said outlet, and means including a thermostat common to all of said units forchanging the control point of each of the last-mentioned thermostats in accordance with changes in the fresh air temperature.

JOHN McELGIN. 

